A variety of mechanical devices are controlled by rotating an input element to a specified angular position. For example, in the heating and ventilating field, valves may be closed or opened by rotating the stem and dampers by adjusting their shutters, perhaps with a crank or eccentric. These devices are usually operated by a reversible electric actuator of some kind. A solenoid is a simple type of a linear actuator. When operating devices such as valves however, it is preferred to use a reversible rotary actuator because of the convenience of having a rotary actuator for a rotary load. Of course, by use of a rack and pinion or crank, rotary motion can be converted to linear motion and vice versa.
The internal construction of these rotary actuators is quite simple. A small reversible electric motor drives the output shaft through a reduction gear train. The ratio and internal friction of the gear train is usually such that the output shaft is locked against movement caused by back torque from the load. There is also some sort of adjustable stop or limit switch to define each end of the output shaft rotation.
In the course of long term system operation, there are many short and long term power outages. In certain circumstances, there are safety implications if the electric power for the control unit for the system of which an actuator forms a part should fail. For example, if a gas valve is open at the time power is lost, it is important that the valve be promptly closed to prevent accumulation of dangerous quantities of gas. There are many other reasons also why it is necessary or desirable to close or operate a device upon loss of electric power.
To answer these requirements, a class of actuators having a spring return feature have been developed. The idea is that when power is lost to the drive motor, the spring will generate sufficient torque on the output shaft to return its load to the safety position. In the simplest embodiments, the spring is wound and unwound as the motor drives the output shaft away from and toward the safety position during normal operation of the system. When power fails, a brake is released and the spring drives both motor and load to the safety position. This mean that the motor must be sized to overcome the spring torque during outward excursions of the output shaft. Since reversible electric motors typically provide the same torque regardless of the direction in which they are driving this means that the motor must have torque output substantially greater than that required merely to move the load.